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<Title>Boost Graph Library: Dijkstra Visitor</Title>
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<H1><img src="figs/python.gif" alt="(Python)"/>Dijkstra Visitor Concept</H1>

This concept defines the visitor interface for <a
href="./dijkstra_shortest_paths.html"><tt>dijkstra_shortest_paths()</tt></a>
and related algorithms. The user can create a class that matches this
interface, and then pass objects of the class into
<tt>dijkstra_shortest_paths()</tt> to augment the actions taken during
the search.

<h3>Refinement of</h3>

<a href="../../utility/CopyConstructible.html">Copy Constructible</a>
(copying a visitor should be a lightweight operation).


<h3>Notation</h3>

<Table>
<TR>
<TD><tt>V</tt></TD>
<TD>A type that is a model of Dijkstra Visitor.</TD>
</TR>

<TR>
<TD><tt>vis</tt></TD>
<TD>An object of type <tt>V</tt>.</TD>
</TR>

<TR>
<TD><tt>G</tt></TD>
<TD>A type that is a model of Graph.</TD>
</TR>

<TR>
<TD><tt>g</tt></TD>
<TD>An object of type <tt>G</tt>.</TD>
</TR>

<TR>
<TD><tt>e</tt></TD>
<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::edge_descriptor</tt>.</TD>
</TR>

<TR>
<TD><tt>s,u,v</tt></TD>
<TD>An object of type <tt>boost::graph_traits&lt;G&gt;::vertex_descriptor</tt>.</TD>
</TR>

<TR>
<TD><tt>DistanceMap</tt></TD>
<TD>A type that is a model of <a href="../../property_map/doc/ReadWritePropertyMap.html">Read/Write Property Map</a>.</TD>
</TR>

<TR>
<TD><tt>d</tt></TD>
<TD>An object of type <tt>DistanceMap</tt>.</TD>
</TR>

<TR>
<TD><tt>WeightMap</tt></TD>
<TD>A type that is a model of <a href="../../property_map/doc/ReadWritePropertyMap.html">Readable Property Map</a>.</TD>
</TR>

<TR>
<TD><tt>w</tt></TD>
<TD>An object of type <tt>WeightMap</tt>.</TD>
</TR>

</table>

<h3>Associated Types</h3>

none
<p>

<h3>Valid Expressions</h3>

<table border>
<tr>
<th>Name</th><th>Expression</th><th>Return Type</th><th>Description</th>
</tr>

<tr>
<td>Initialize Vertex</td>
<td><tt>vis.initialize_vertex(u, g)</tt></td>
<td><tt>void</tt></td>
<td>
This is invoked one each vertex of the graph when it is initialized.
</td>
</tr>

<tr>
<td>Examine Vertex</td>
<td><tt>vis.examine_vertex(u, g)</tt></td>
<td><tt>void</tt></td>
<td>
This is invoked on a vertex as it is popped from the queue. This
happens immediately before <tt>examine_edge()</tt> is invoked
on each of the out-edges of vertex <tt>u</tt>.
</td>
</tr>

<tr>
<td>Examine Edge</td>
<td><tt>vis.examine_edge(e, g)</tt></td>
<td><tt>void</tt></td>
<td>
This is invoked on every out-edge of each vertex after it is discovered.
</td>
</tr>

<tr>
<td>Discover Vertex</td>
<td><tt>vis.discover_vertex(u, g)</tt></td>
<td><tt>void</tt></td>
<td>
This is invoked when a vertex is encountered for the first time.
</td>
</tr>

<tr>
<td>Edge Relaxed</td>
<td><tt>vis.edge_relaxed(e, g)</tt></td>
<td><tt>void</tt></td>
<td>
Upon examination, if the following condition holds then the edge
is relaxed (its distance is reduced), and this method is invoked.<br>
<tt>
tie(u,v) = incident(e, g);<br>
D d_u = get(d, u), d_v = get(d, v);<br>
W w_e = get(w, e);<br>
assert(compare(combine(d_u, w_e), d_v));<br>
</tt>
</td>
</tr>

<tr>
<td>Edge Not Relaxed</td>
<td><tt>vis.edge_not_relaxed(e, g)</tt></td>
<td><tt>void</tt></td>
<td>
Upon examination, if the edge is not relaxed (see above) then
this method is invoked.
</td>
</tr>

<tr>
<td>Finish Vertex</td>
<td><tt>vis.finish_vertex(u, g)</tt></td>
<td><tt>void</tt></td>
<td>
This invoked on a vertex after all of its out edges have been added to the
search tree and all of the adjacent vertices have been discovered
(but before their out-edges have been examined).
</td>
</tr>

</table>

<h3>Models</h3>

<ul>
 <li><a href="./dijkstra_visitor.html"><tt>dijkstra_visitor</tt></a>
</ul>

<a name="python"></a>
<h3>Python</h3>

To implement a model of the <tt>DijkstraVisitor</tt> concept in Python,
create a new class that derives from the <tt>DijkstraVisitor</tt> type of
the graph, which will be
named <tt><i>GraphType</i>.DijkstraVisitor</tt>. The events and syntax are
the same as with visitors in C++. Here is an example for the
Python <tt>bgl.Graph</tt> graph type:

<pre>
class count_tree_edges_dijkstra_visitor(bgl.Graph.DijkstraVisitor):
  def __init__(self, name_map):
    bgl.Graph.DijkstraVisitor.__init__(self)
    self.name_map = name_map

  def edge_relaxed(self, e, g):
    (u, v) = (g.source(e), g.target(e))
    print "Relaxed edge ",
    print self.name_map[u],
    print " -> ",
    print self.name_map[v]
</pre>

<br>
<HR>
<TABLE>
<TR valign=top>
<TD nowrap>Copyright &copy; 2000-2001</TD><TD>
<A HREF="http://www.boost.org/people/jeremy_siek.htm">Jeremy Siek</A>,
Indiana University (<A
HREF="mailto:jsiek@osl.iu.edu">jsiek@osl.iu.edu</A>)<br>
<A HREF="http://www.boost.org/people/liequan_lee.htm">Lie-Quan Lee</A>, Indiana University (<A HREF="mailto:llee@cs.indiana.edu">llee@cs.indiana.edu</A>)<br>
<A HREF="https://homes.cs.washington.edu/~al75">Andrew Lumsdaine</A>,
Indiana University (<A
HREF="mailto:lums@osl.iu.edu">lums@osl.iu.edu</A>)
</TD></TR></TABLE>

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